A quantum leap is in the works for secure cloud computing

Clusters of entangled qubits, shown in this artistic visualization, could allow remote quantum computing to be performed on a server while keeping the contents and results hidden from the remote server.

If the future is heading toward "cloud computing," where most of your data lives on someone else's server, can you trust the cloud to keep a secret? Researchers say they've found a way to guarantee that your information will be secure in the cloud, using quantum entanglement.

The technique is called blind quantum computing, and it adds one more piece to a puzzle that could eventually be assembled into an entirely new infrastructure for data processing. Theoretically, quantum computers could outdo classical computers when it comes to making weather predictions, simulating biological processes, analyzingchemical reactions and, not incidentally, deciphering secret codes. Data security could become an even bigger issue than it is today.

Whom do you trust?Today, most of your computing power probably resides on the device you're using, whether it's a desktop or a smartphone. If you send secure data someplace else, those bits are probably encrypted using classical mathematical techniques. They're tough codes to break, but they're not unbreakable. In fact, computer scientists say quantum computers might be well-suited for cracking today's classical codes.

At the same time, there's a trend toward developing devices that shift more of the computing power onto big servers. You would still use your tablet or smartphone or netbook for input and output, but the information is stored and processed as part of a huge cloud of bits on the server. That's the idea behind the much-debated cloud computing approach.

How sure can you be that the folks who manage the cloud won't meddle with your data? And could a malicious cloud client meddle with the central server? Such questions are tricky now, and they could get trickier if quantum computing takes hold, according to an international research team led by Stefanie Barz of the Vienna Center for Quantum Science and Technology at the University of Vienna and the Austria-based Institute for Quantum Optics and Quantum Information.

In this week's issue of the journal Science, Barz and her colleagues note that quantum computers will be so complex that there may be only a few of them in operation at specialized facilities around the world.

"A key challenge in using such central quantum computers is enabling a quantum computation on a remote server while keeping the client's data hidden from the server," they write.

Demonstrating blind computingThe researchers worked out a system to entangle photons of light that were generated by a nonlinear crystal, and then "process" those entangled photons on an experimental setup of beam splitters, filters and couplers. The photons served as quantum bits, or qubits, to be manipulated in two types of quantum calculations (Deutsch's algorithm and Grover's search).

In this scenario, the person who provided the qubits knows their initial entangled state, and can thus decipher the entangled outcome. But the company that does the data processing wouldn't know how the qubits were entangled — and thus could not even try to decode the qubits without essentially destroying them. As far as it's concerned, all those qubits look like a totally random hodgepodge. What's more, the system has a built-in verification scheme.

"By inspecting the output, you can know if the company really has a quantum computer, without disclosing your algorithm, the input, or indeed the output," the University of Oxford's Vlatko Vedral said in a Science commentary on the research. "The computation is thus 'doubly' blind."

Barz and her colleagues say there are still some technical challenges to be overcome. for example, it's theoretically possible for some of the photons emitted while preparing the qubits to reveal information about the "blind" phase of the calculation. Also, it's important to have a high-fidelity, low-signal-loss system for processing the qubits — whether they consist of photons with different polarizations, or electrons with different spins. But however the quantum computing puzzle is put together, the researchers say their experiments will have contributed a key piece.

"Our demonstration is crucial for unconditionally secure quantum cloud computing," they say, "and might become a key ingredient for real-life applications, especially when considering the challenges of making powerful quantum computers widely available."